1. Field
The present invention relates generally to communication systems, and more specifically to a method and an apparatus for enabling subscriber stations to process a synchronization channel in a communication system.
2. Description of Related Art
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication channel bandwidth. At the destination station the original information signal is replicated from the modulated carrier wave received over the communication channel. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of remote subscriber units requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA) and frequency division multiple-access (FDMA). Another type of a multiple-access technique is a code division multiple-access (CDMA) spread spectrum system that conforms to the “TIA/EIA/IS-95 Subscriber station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
A multiple-access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of a communication systems carrying both voice and data comprise communication systems conforming to the “3rd Generation Partnership Project” (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard).
In a multiple-access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one mobile subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.
Each of the standards noted above defines a mechanism for initiating and processing a call between a subscriber station and a base station. The mechanism is characterized by a call processing state machine on the signaling layer (i.e., layer-3) that includes a number of states and a set of allowed transitions between the states. Each state in the state machine corresponds to a particular state of the subscriber station (or base station) with respect to the call being processed. A transition to a new state takes place upon the occurrence of certain specified events.
An example of a state machine 100, embodying a subscriber station call processing is shown in FIG. 1. A detailed description of the state machine is described in the applicable CDMA standards (e.g., the IS-95 and IS-2000 standards), the following description is for tutorial purposes only.
As shown in FIG. 1, upon power-up, the subscriber station transitions from a power-up state 110 to a subscriber station initialization state 112. In state 112, the subscriber station selects a particular system to use. If the selected system is an analog system, e.g., an Advanced Mobile Phone System (AMPS), the subscriber station transitions to a state 114 and begins analog mode operation. Otherwise, if the selected system is a CDMA system, the subscriber station reads a frequency (a primary frequency) from the subscriber station's memory, and attempts to acquire a pilot channel transmitted on the primary frequency by one or more base stations in the selected system. If the attempt is unsuccessful, the subscriber station reads another frequency (a secondary frequency) from the subscriber station's memory, and attempts to acquire the pilot channel transmitted on the secondary frequency. Therefore, the primary and the secondary frequencies may be, for the purposes of this document, referred to as system access frequencies. The Once the subscriber station has acquired the pilot channel, the subscriber station acquires a synchronization channel transmitted on the same frequency that provides the subscriber station with timing of the selected CDMA system. The synchronization channel further provides the subscriber station with messages, affecting further actions taken by the subscriber station. Among the messages, the subscriber station is provided with a Sync Channel Message. In addition to primary and secondary frequencies, there the phone can also contain a roaming list, with one or more frequencies. The roaming list may comprise the primary and secondary frequencies as well as additional frequencies. Roaming lists are known in the art, and are described in TIA/EIA/IS-683, “Over-the-Air Service Provisioning of Mobile Stations in Spread Spectrum Systems.”
FIG. 2 illustrates an example of a Sync Channel Message 200 in accordance with the IS-2000 standard, which provides definitions and explanation of the fields comprising the message. The first portion 202 of the Sync Channel Message 200 comprises information intended for subscriber station operating in accordance with a revision 5 (P_REV 5) of the IS-95 standard. The subscriber stations operating in accordance with a revision 5 of the IS-95 standard (hereinafter P_REV 5 subscriber stations) read only fields of the first portion 202, and ignore the fields in the second portion 204 of the Sync Channel Message 200. The second portion 204 of the Sync Channel Message 200, comprises additional information intended for subscriber station operating in accordance with revisions 6 and 7 of the IS-2000 standard (hereinafter P_REV 6 and P_REV 7 subscriber stations).
Referring back to FIG. 1, the Sync Channel Message contains a field, (CDMA_FREQ for P_REV 5 subscriber stations and EXT_CDMA_FREQ for P_REV 6 and P_REV 7 subscriber stations) providing a subscriber station with a frequency, to which the subscriber station is to tune. The subscriber station tunes to the frequency specified by the appropriate field, and acquires a paging channel at the frequency. Upon acquiring the paging channel, the subscriber station registers with the sector. The paging channel further provides the subscriber station with messages, affecting further actions taken by the subscriber station. Among the messages, the subscriber station is provided with a message, (CDMA Channel List Message) which contains a list of frequencies to which the subscriber station is to tune. The subscriber station uses a hash function to select one of the frequencies, and tunes to the selected frequency. The subscriber station then acquires a paging channel transmitted at the selected frequency, and it enters a subscriber station idle state 116.
In state 116, the subscriber station monitors the paging channel on the forward link for messages from the base station. If the subscriber station is unable to receive the paging channel or if the mobile station moves to the coverage of another base station, the mobile station acquires the other base station and performs an idle handoff. In some cases, the subscriber station returns to state 112 while performing this idle handoff. In state 116, the subscriber station can receive messages or an incoming call, originate a call, perform registration, initiate a message transmission, or perform some other actions. Upon initiating any of these actions, the subscriber station transitions to a system access state 118.
In state 118, the subscriber station sends messages to the base station on one or more access channels and receives messages from the base station on the paging channel in an attempt to access the base station. The exchange of messages is dependent on the particular type of communication (e.g., voice, data) between the subscriber station and the base station and the originator of the message (i.e., the subscriber station or base station). Depending on the outcome of the message exchange, the subscriber station can return to idle state 116 if no communication on a traffic channel is to be performed with the base station or proceed to a subscriber station control on the traffic channel state 120 if a call with the base station is to be processed. Before the transition to state 120, the subscriber station is assigned a forward traffic channel for the call.
In state 120, the subscriber station communicates with the base station using the established forward and reverse traffic channels. Upon termination of the call, the subscriber station returns to state 112.
Due to a design/manufacturing error, certain P_REV 5 subscriber stations fail to ignore the second portion 204 of the Sync Channel Message 200 (of FIG. 2), which causes a failure of the P_REV 5 subscriber stations' operation. The problem can manifest itself in a communication system comprising sectors capable of supporting communication with subscriber stations operating in accordance with both IS-95 and IS-2000 standards. An exemplary frequency and channel assignment for such a sector is illustrated in Table I. One skilled in the art understands that all illustrations of frequency and channel assignment for a sector are for tutorial purposes only, and can be modified to suit a particular implementation of a communication system.
TABLE 1FREQUENCYCHANNELf1Synchronization channel (F-SYNC)Paging channel (F-PCH)f2Paging channel (F-PCH)f3Broadcast Control Channel (F-BCCH)Common Control Channel (F-CCCH)f4Broadcast Control Channel (F-BCCH)Common Control Channel (F-CCCH)
The sector transmits a Synchronization Channel (F-SYNC) and a Paging Channel (F-PCH) on the forward link on the primary frequency f1. For the sake of simplicity, as illustrated in Table I the primary frequency is also the secondary frequency. However, one of ordinary skills in the art understands that this is for tutorial purposes only, and the sector may comprise another frequency with the same channel assignments as frequency f1 designated as a secondary frequency. The sector further transmits a F-PCH on the forward link on the frequency f2. The subscriber station operating in accordance with both IS-95 and IS-2000 standards can process the F-SYNC and the F-PCH; consequently, the frequencies f1 and f2 can support both types of the subscriber stations. The sector transmits a Broadcast Control Channel (F-BCCH) and a Common Control Channel (F-CCCH) on the forward link on frequencies f3 and f4. Because the F-BCCH and the F-CCCH replaced the F-PCH for P_REV 7 subscriber stations, the frequencies f3 and f4 can support only the P_REV 7 or higher protocol revision subscriber stations. Note that forward pilot channel and other forward channels transmitted by the sector are not illustrated for the sake of simplicity.
Referring back to FIG. 1, and accompanying text for reference, all subscriber stations read the primary frequency f1 from its memory or from the roaming lists held within its memory and acquire the forward pilot channel on frequency f1. All subscriber stations then acquire the F-SYNC and read the Sync Channel Message. A subscriber's station memory may consist of its physical memory or memory located on an external, but connected device such as a SIM (Subscriber Identification Modulate), also called a UIM (User Identification Module).
The subscriber stations operating in accordance with the IS-2000 standard tune to a frequency provided in the field EXT_CDMA_FREQ, and acquire the F-BCCH and the F-CCCH transmitted on the frequency. Because the F-BCCH and the F-CCCH, which the mobiles can receive, is transmitted on frequencies f3 and f4, the EXT_CDMA_FREQ field contains either frequency f3 or frequency f4. The F-BCCH provides the subscriber stations with an Extended CDMA Channel List Message. The subscriber stations use a hash function to select one of the frequencies contained in the Extended CDMA Channel List Message. The field CDMA_FREQ identifies each of these frequencies in the message. Because the F-BCCH and the F-CCCH, which the mobiles can receive, is transmitted on frequencies f3 and f4, the list of frequencies contains both frequencies f3 and f4. Due to the operation of the hash function, approximately half of the subscriber stations will be directed to frequency f3 (hereinafter hashed to frequency) and the remaining subscriber stations will be hashed to frequency f4. The subscriber stations then acquire the F-BCCH and the F-CCCH transmitted on the respective hashed to frequency f3 and f4.
The subscriber stations operating correctly in accordance with the IS-95 standard read the first portion (202 of FIG. 2) of the Sync Channel Message 200, tune to a frequency provided in the field CDMA_FREQ, and acquire the F-PCH transmitted on the frequency. Because the F-PCH, which the mobiles can receive, is transmitted on frequencies f1 and f2, the CDMA_FREQ field contains either frequencies f1 or f2. The F-PCH provides the subscriber stations with the CDMA Channel List Message and may provide the Extended CDMA Channel List Message. The subscriber stations operating in accordance with the IS-95 standard ignore the Extended CDMA Channel List Message and use a hash function to select one of the frequencies contained in the CDMA_FREQ field of the CDMA Channel List Message. Because the F-PCH, which the mobiles can receive, is transmitted on frequencies f1 and f2, the CDMA_FREQ field contains both frequencies f1 and f2. Due to the operation of the hash function, the subscriber stations will be approximately evenly distributed among frequencies f1 and f2. The subscriber stations then acquire the F-PCH transmitted on the respective hashed to frequency f1 and f2.
The subscriber stations operating in accordance with the IS-95 standard, but containing the above-mentioned flaw, read the Sync Channel Message past the first portion (202 of FIG. 2) and cease to operate properly.